I'm having problems with this and was looking for some insight.

"Randy is going to use a porcelain bowl to measure rain. The bowl has the shape $\displaystyle z=x^2+y^2$ from $\displaystyle z=0$ to $\displaystyle z=10$ inches. Randy must calibrate the bowl so that he can precisely measure the amount of rain that falls.Are there any ideas?What height of water in the bowl corresponds to an inch of rain?That is to say, a circular cylinder would have one inch of rain in it, or that the level ground would have standing water one inch deep."